The present invention relates to internal combustion engines. More particularly, the present invention relates to engine valve actuation.
It is desirable that a hydraulically-assisted engine valve actuator provide for flexible engine valve operation under a wide band of engine operating conditions. The hydraulically-assisted engine valve actuator should provide for variable valve timing of closing and opening and variable lift as desired in order to achieve the greatest engine efficiencies. Presently, hydraulic fluid is supplied to hydraulically actuated valves through tubes commonly called rails. Valve motion profiles in current hydraulic actuation designs depend on a pre-established constant value of oil pressure at the supply rails because rail pressures cannot be adjusted fast enough to modulate valve profiles. The constant rail pressure values result in constant valve profiles regardless of engine rpm.
Present hydraulic actuation schemes add complexity to the engine design. Some hydraulic actuation designs rely on additional hydraulic supply rails at constant pressure levels. Further, hydraulic actuation that relies on on/off solenoid (spool or poppet) valve operations require engine valve position sensors for reliable timing of the solenoids and for safe operation. The plurality of sensors required, further adds to the engine complexity.
A hydraulically-assisted engine valve actuator should provide for uniform valve actuation over a wide range of hydraulic fluid temperatures. Present hydraulic actuation schemes typically rely on mechanical damping mechanisms for seating in order to prevent the valve from seating too rapidly. Such mechanisms are typically very dependent on oil temperature, leading to nonuniform valve actuation characteristics.
There is further a need to ensure the opening of an engine exhaust valve, especially under conditions of very high compression forces in the combustion chamber of the engine. Such conditions occur, for example, during compression braking of the engine. Where hydraulic actuation is utilized for such exhaust valve opening, it is important to minimize the volume of hydraulic actuation fluid that is necessary to effect the valve opening.
The hydraulically-assisted engine valve actuator of the present invention allows for flexible engine valve operation: variable valve timing of the closing and the opening and variable valve lift. Further, the mechanical components needed to effect the hydraulic actuation are relatively simple, thereby minimizing the additional engine components required. No sensors or mechanical damping mechanisms are needed. Additionally, the hydraulic actuation of the present invention is designed to provide for uniform actuation over a wide range of hydraulic fluid temperatures and viscosities.
The foregoing advantages of the present invention are effected by the use of fine needle control. The fine needle control provides for modulation of engine valve profiles: varying engine profiles at varying engine speeds, varying the shape of the profiles at a given rpm. The present invention further allows aggressive valve openings and closings which translates into better volumetric efficiency of the engine.
The hydraulically-assisted engine valve actuator of the present invention is not sensitive to pressure variation in the high-pressure rail, that is, the modulation of engine valve motion is capable of tolerating a substantial variation of pressure (above a predetermined threshold pressure) in the high-pressure rail.
The low-pressure line in an embodiment of the present invention is shared with the existing lubricating oil supply. In an embodiment of the present invention, only a high-pressure supply line is required. Spent hydraulic actuation fluid is vented to the engine oil pan or reservoir. In the case of engines with a fuel injection system incorporating a high-pressure rail, the same high pressure fluid supply is used for valve actuation in order to further minimize the added components to the engine.
In the case of the present invention, the output, i.e. the engine valve position, very closely follows the input to the hydraulic actuator. Therefore, the device of the present invention does not require the added complexity of requiring a sensor to measure engine valve position for feedback control. Accurate control of valve seating is attained by accurate control of the needle at the end of the stroke.
The present invention further provides very good cold temperature operating performance despite the hydraulic actuating fluid preferably being lubricating oil. The proportional flow areas of the hydraulic fluid passages are not so small as to compromise performance under variable operating temperatures. This is especially important in cold temperature operation since the viscosity of hydraulic fluid, particularly lubricating oil, is significantly higher when the engine is cold than after the engine has warmed up.
In one embodiment, the invention incorporates a power piston to assist in opening the exhaust valve of the engine. The power piston operates with each cycle of the engine valve and does not require a separate valve that is dedicated to control of the power piston, as is the case with known power pistons. Additionally, the stroke of the power piston is limited to that necessary to only crack open the exhaust valve. Once the exhaust valve is cracked open, the compressive forces in the combustion chamber are relieved and the servo piston alone can complete the full opening of the exhaust valve without the assistance of the power piston. By limiting the stroke of the power piston, the volume of high pressure actuating fluid necessary to activate the power piston is minimized.
Further, the mechanical components that are required for valve actuation by the present invention do not significantly increase the engine complexity, i.e., very few modifications to an existing cylinder head are needed in order to incorporate the valve actuator assembly of the present invention.